Narrow QRS Complexes in a Patient with Baseline Surgical RBBB: What is the Mechanism?

A 26-year-old woman born with Tetralogy of Fallot initially underwent a left Blalock-Taussig shunt followed by a complete repair with a transannular patch at 2 years of age. She complained of non-sustained short episodes of palpitations. Her Holter monitor showed episodes of six beat run of non-sustained ventricular tachycardia. She was scheduled to undergo surgical pulmonary valve replacement. In view of her symptoms and Holter results she underwent an electrophysiology study to map any substrate of ventricular tachycardia for intra-operative ablation. Her baseline electrocardiogram demonstrated complete right bundle branch block pattern (RBBB) with QRS duration of 174 msec secondary to surgical interruption of the right bundle branch. During the electrophysiology study she was noted to have intermittent narrow QRS complexes (Figure 1).The QRS complexes had a duration of 98 msec with no change in the PR interval and AH interval. What is the mechanism of narrow QRS complexes in this patient with baseline right bundle branch block (RBBB) pattern?